Nuclear magnetic relaxation studies of BH4 in metal Borohydrides

A different approach of nuclear magnetic resonance (NMR) relaxation of the rotational motion of BH4 tetrahedron in light metal borohydrides was introduced. Previous NMR relaxation studies were mainly performed using an approximation which is based on a single exponential transition approach for the spin-lattice relaxation of the spin under quadrupolar interaction. In this new approach, a relaxation rate equation was derived by strictly taking into account both electric quadrupolar and magnetic dipolar contributions to the transitions between Zeeman energy states of quadrupolar B-11 nucleus that was coupled to many proton spins. Two exponential relaxation times derived from our theoretical treatment were applied to the analysis of B-11 spin-lattice relaxation of LiBH4 and Ca(BH4)(2). The activation energies and the correlation times of the reorientational motions of BH4 were evaluated from the two experimental spin-lattice relaxation times. The results were compared to previous activation energies and correlation times.